I think I'll probably need about 3. My HE washer gets things wrung out so well that I'm already down to 30-40 minutes for dry time (for denim!) from the 75 it used to be, so I don't think I'll need too many to cut the dry time down to 15-30 min.

I think I'll need 4 wool dryer balls. I'm really not sure because I've never used dryer balls before. I also have a small family. I usually like to hang dry my baby girls flats, I always love how soft and fluffy they are when I machine dry them!

AN ANSWER! I've been struggling with static in my MF inserts and even considered buying new inserts but now I see I need to invest in wool balls. For diapers I'd probably only need 2 but I'd love to use these with my towels too so I'd invest in at least 4-6.